ZipA and FtsA are recruited independently to the FtsZ cytokinetic ring (Z ring) and are essential for cell division of Escherichia coli . The molecular role of FtsA in cell division is unknown; however, ZipA is thought to stabilize the Z ring, anchor it to the membrane, and recruit downstream cell division proteins. Here we demonstrate that the requirement for ZipA can be bypassed completely by a single alteration in a conserved residue of FtsA (FtsA*). Cells with ftsA * in single copy in place of WT ftsA or with ftsA * alone on a multicopy plasmid divide mostly normally, whether they are zipA + or zipA −. Experiments with ftsQAZ and ftsQA * Z on multicopy plasmids indicate that ftsQAZ / zipA + and ftsQA * Z / zipA − cells divide fairly normally, whereas ftsQAZ / zipA − cells divide poorly and ftsQA * Z / zipA + cells display a phenotype that suggests their septa are unusually stable. In support of the idea that ftsA * stabilizes Z rings, single-copy ftsA * confers resistance to excess MinC, which destabilizes Z rings. The inhibitory effect of excess ZipA on division is also suppressed by ftsA *. These results suggest that the molecular mechanism of the FtsA* bypass is to stabilize FtsZ assembly via a parallel pathway and that FtsA* can replace the multiple functions of ZipA. This is an example of a complete functional replacement of an essential prokaryotic cell division protein by another and may explain why most bacteria can divide without an obvious ZipA homolog.

The MinCDE proteins help to select cell division sites in normal cylindrical Escherichia coli by oscillating along the long axis, preventing unwanted polar divisions. To determine how the Min system might function in cells with multiple potential division planes, we investigated its role in a round‐cell rodA mutant. Round cells lacking MinCDE were viable, but growth, morphology and positioning of cell division sites were abnormal relative to Min+ cells. In round cells with a long axis, such as those undergoing cell division, green fluorescent protein (GFP) fusions to MinD almost always oscillated parallel to the long axis. However, perfect spheres or irregularly shaped cells exhibited MinD movement to and from multiple sites on the cell surface. A MinE–GFP fusion exhibited similar behavior. These results indicate that the Min proteins can potentially localize anywhere in the cell but tend to move a certain maximum distance from their previous assembly site, thus favoring movement along the cell's long axis. A new model for the spatial control of division planes by the Min system in round cells is proposed.